The complexation of a series of new bis(meta-pyridyl)methylsiloxane ligands with palladium(II) acetate in dilute toluene-d 8 solution was studied by 1H NMR at 233−363 K, measuring the binding affinity and ring−chain distribution as a function of ligand structure, temperature, and concentration. Significant differences in the ring distribution and Pd binding affinity were observed as a function of siloxane chain length, with hexa- and heptasiloxane spacers binding Pd most effectively. The 1H NMR resonance of the singlet ortho aromatic proton was shifted unusually far downfield for the bidentate complex; the effect was strongest for the shortest chains and disappeared for chains with 10 or more siloxane units. Fitting the experimental data to a modified Jacobson−Stockmayer model demonstrated that the bidentate bis(pyridyl)siloxane complexes function as chelates in the concentration range 1−10 mM. Variable-temperature studies showed that all of the observed cyclic coordination oligomers are strainless. Therefore, entropic effects are implicated as the main cause of the observed trends in binding affinity, particularly the loss of conformational freedom of the siloxane chain upon binding.

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Grafting of pairs of functional groups onto a silica surface was demonstrated by tethering both terminals of an organochlorosilane precursor molecule, Cl 2 (CH 3)Si(CH 2) 4 (CO)(OSi(i-Pr) 2)(CH 2) 2 Si(CH 3)Cl 2 , that possess a cleavable silyl ester bond, onto a silica surface. Hydrolytic cleavage of the silyl ester bond of the grafted molecule resulted in the generation of organized pairs of carboxylic acid and organosilanol groups. This organosilanol moiety was easily transformed into other functional groups through condensation reactions to form, together with the neighboring acid group, pairs such as carboxylic acid/secondary amine, carboxylic acid/pyridine, and carboxylic acid/phosphine. In the case of carboxylic acid/amine pairing, there was evidence of the formation of amide. A sample grafted with amine-carboxylic acid pairs was three times more active (per free amine) than a sample without such pairs for the nitroaldol condensation of 4-nitrobenzaldehyde and nitromethane.